1. Technical Field
This invention relates to surface emitting semiconductor lasers that emit laser light in a direction normal to the surface of the laser substrate and, further, relates to the manufacturing process of a surface emitting semiconductor laser.
2. Background Technology
We have previously proposed a surface emitting semiconductor laser with a buried optical resonator employing a Group II-VI compound semiconductor materials set forth in Japanese patent application No. 2-242000. This surface emitting semiconductor laser is illustrated in FIG. 13 and comprises an n-GaAs buffer layer 403, a distributed Bragg reflection type multiple layer mirror 404, an n-Al.sub.0.4 Ga.sub.0.6 As cladding layer 405, a p-GaAs active layer 406, a p-Al.sub.0.4 Ga.sub.0.6 As cladding layer 407, and a p-Al.sub.0.4 Ga.sub.0.9 As contact layer 408. These layers are sequentially grown on an n-GaAs substrate 402. The p-Al.sub.0.4 Ga.sub.0.6 As cladding layer 407 and the p-Al.sub.0.1 G.sub.0.9 As contact layer 408 are selectively etched leaving a columnar shaped region. Then, a ZnS.sub.0.06 Se.sub.0.94 confinement layer 409 is formed in the removed regions of these two layers around the circumference of the column shaped region. Lastly, a p-type ohmic electrode 410 and a n-type ohmic electrode 401 are deposited on the top and bottom surfaces of the laser structure. After this, multi-layer mirror 411 is deposited in a region that is slightly smaller than the diameter of the column shaped region on the surface of p-Al.sub.0.1 Ga.sub.0.9 As contact layer 408.
Since ZnS.sub.0.06 Se.sub.0.94 confinement layer 409, employed as a burying layer has a high resistance and low refractive index, the efficient confinement of current and light can take place, creating a high performance surface emitting semiconductor laser.
However, there are problems remaining with this technology. The flow of current in active layer 406 takes place through the distributed Bragg reflection mirrors. The distributed Bragg reflection mirrors have an AlGaAs layer with a large energy bandgap and a high aluminum content. Also, this layer may be comprised of alternately stacked layers of AlGaAs that have a large bandgap with a large aluminum content and a small bandgap with a low aluminum content. This results in a structural energy band that is not continuous and has high resistance rendering it difficult for the flow of current. As a result, the elemental resistance of the surface emitting semiconductor laser increases as the laser heats up through operation, resulting in inadequate reliability of the laser with an increase in the threshold current and making high speed modulation of the laser difficult.
To resolve these problems, we proposed a high concentration of dopant to the entire distributed Bragg reflection mirror in order to reduce the laser resistance. However, this resulted in the creation of a new problem. The film quality of the layer that comprises the distributed Bragg reflection mirror deteriorated and the laser characteristics were lost.
Tai, et al, in an article entitled "Drastic Reduction of Series Resistance in Doped Semiconductor Distributed Bragg Reflectors for Surface-Emitting Lasers", Applied Physics Letter, No. 56, pp. 2496-2498, Jun. 18, 1990 have attempted to resolve the same type of problem by fabricating an intermediate layer with an intermediate energy bandgap formed between the energy bandgaps of the two types of layers that form the distributed Bragg reflection mirrors. However, when we examined this approach, the following problems were encountered:
(1) Because three or more layers with different compositions have to be sequentially formed forming a stacked distributed Bragg reflection mirror, the manufacturing process becomes complicated, causing a reduction in yield and a lack of uniform properties.
(2) By fabricating a layer that has an intermediate bandgap, the hetero-barrier of the energy band is relieved. However, because the inclusion of this intermediate layer concurrently steepens the refractive index distribution among the distributed Bragg reflection mirrors with the same number of layers, there is a corresponding reduction in the reflection power of the mirror with the intermediate layer. This causes a deterioration in operating properties of the laser, for example, a rise in its threshold current.
A purpose of this invention is to resolve these foregoing problems.
It is an objective of this invention to offer a surface emitting semiconductor laser that is highly efficient and reliable.
It is a further object of this invention to provide a manufacturing process for fabricating a surface emitting laser utilizing simplified manufacturing steps.